The speed of sound in air varies with the temperature of the air. At 59 degrees Fahrenheit, the speed of sound is 761.2 mph, while an air temperature of 68 degrees Fahrenheit increases the speed of sound to 767 mph.
Generally, the higher the ambient air temperature, the higher the speed of sound, and vice versa. This is the reason why high-flying aircraft can surpass the speed of sound at a relatively low speed. The air temperature at high elevations drops considerably, up to a point. At approximately 12.5 miles from the surface of the planet, the ozone layer of the stratosphere causes the air to heat up, increasing the speed of sound.